1. Field of the Invention
The present invention relates to a cathode-ray tube (CRT) which has a plurality of electron beams, and in particular, to a correction system for correcting vertical misconvergence which appears to be unsymmetrical upwardly and/or downwardly with respect to a center beam.
In a cathode-ray tube according to the present invention which has a plurality of electron beams, a uniform magnetic field is selectively applied to a required one of the electron beams with a magnetic member which is electrically coupled to a final electrode of an electron gun so as to correct the vertically unsymmetrical misconvergence without causing distortion in the shape of any of the beam spots.
2. Description of the Prior Art
In cathode-ray tubes which have a plurality of electron beams of an in-line array and which are deflected for scanning a striped color luminescent screen, misconvergence which occurs at a faceplate includes horizontal misconvergence and vertical misconvergence. FIG. 8A shows the normally converged state of electron beams 1R, 1G, and 1B which are respectively associated with red, green and blue on the luminescent screen. The horizontal misconvergence includes a horizontally symmetrical misconvergence in which the side beams 1R and 1B deviate from the center beam 1G, as shown in FIG. 8D, equidistantly and symmetrically in the horizontal direction and a horizontally unsymmetrical misconvergence in which a beam, for example, the beam 1B on one side deviates from the center beam 1G in the horizontal direction as shown in FIG. 8E.
Also, the vertical misconvergence is also classified into vertically symmetrical misconvergence in which both of the side beams 1R and 1B are unsymmetrically shifted from the center beam 1G in the vertical direction as shown in FIG. 8C. In an ordinary case, the misconvergence results from a combination of the symmetrical or unsymmetrical horizontal misconvergence with the symmetrical or unsymmetrical vertical misconvergence. Consequently, a necessary correction has been made to correct the misconvergence.
The apparatus of the present invention particularly relates to a system for the correction of unsymmetrical vertical misconvergence. In the conventional method of correcting such vertical unsymmetrical misconvergence, the correction has been commonly achieved by means of a six-pole magnet 2 disposed, for example, outside of a tube body at a location associated with the final electrode of an electron gun as shown in FIG. 7A. In this configuration, when a magnetic field is applied from the six-pole magnet 2, forces F1 and F2 are exerted on the side beam 1B, which is accordingly displaced downwardly as indicated by the dotted line, whereas forces F3 and F4 are applied to the side beam 1R, which is therefore shifted downwardly as represented by a dotted line, thereby effecting the correction of the unsymmetrical vertical misconvergence of the three beams 1R, 1G, and 1B (FIG. 7B).
In a case, however, where the vertically unsymmetrical misconvergence is corrected by means of the six-pole magnet 2, the beam spots of the side beams 1R and 1B are distorted in shape as shown in FIG. 7B, which causes a deterioration of resolution. In other words, the beam 1B on the left side of FIG. 7A is affected by a divergent action of the force F2 in a +45.degree. direction and is simultaneously affected by a convergent action of the force F1 in a -45.degree. direction, which results in an overfocused condition in the -45.degree. direction. As a consequence, the beam spot is obliquely distorted in shape and has a halo 3. On the other hand, the beam 1R on the right side is caused to have a spot shape which is distorted in the reverse direction. Consequently, the respective spots of the beams 1R, 1G, and 1B are different from each other in shape and hence there arises a problem that the shapes of the beam spots cannot be simultaneously corrected by any known method. Heretofore, it has been impossible to achieve an appropriate correction of the beam spot distortion caused by the six-pole magnet 2. Particularly, in a tube having high precision, the distortion of the beam spot causes considerable deterioration of the resolution.